1. Field of the Invention
The present invention relates to an electric power steering control device and an electric power steering control method, which uses an electric motor to assist a steering force of a steering wheel, in particular, to a countermeasure against heat for the control device.
2. Description of the Related Art
When a motor is driven in a conventional electric power steering control device, a necessary current is controlled to flow to generate a necessary assist torque. Therefore, a countermeasure against heat generation is required for each of sections. Among the sections, a controller includes a large number of components to supply the current to the motor. Therefore, in the controller, a countermeasure against heat has been taken not only by improving heat radiation performance of each of the components but also by limiting the current to variably change a control amount.
As a conventional device, there exists an electric power steering device which uses a limited motor current expressed by an expression more closely corresponding to a heat radiating condition of an actual component based not on an expression of the first or second power of the current but on the combination of the expressions of the first power and the second power of the current (for example, see Japanese Patent No. 4064600). As another conventional device, there exists an electric power steering device which classifies thermal time constants of components into a small thermal time constant and a large thermal time constant in accordance with the components used in a control circuit for supplying a current to the motor and selects a smaller one of two current limiting values to limit the current (for example, see Japanese Patent No. 3605349).
However, the conventional technologies have the following problems.
It is certain that the conventional devices can limit the current, and therefore can carry out a countermeasure against heat. However, the conventional devices put so high priority on safety that there is a tendency to limit the motor current excessively. Therefore, when a high current flows as the motor current, the motor current is limited early. Then, even after the motor current is reduced or the driving is stopped, the limitation of the motor current is continued for a relatively long period of time, which leads to reduced assist of the steering force.